This invention relates to the retorting of hydrocarbon-containing solids, particularly shale.
Shale oil is not a naturally occurring product, but is formed by the pyrolysis or distillation of organic matter, commonly called kerogen, formed in certain shale-like rock. The organic material has limited solubility in ordinary solvents and therefore cannot be recovered by extraction. Upon strong heating, the organic material decomposes into a gas and liquid. Residual carbonaceous material typically remains on the retorted shale.
In its basic aspects, the retorting of shale oil and also other similar solid hydrocarbon-containing solids is a simple operation. The major step involves the heating of the solid material to the proper temperature and the recovery of the vapor evolved. However, for a commercially feasible process it is necessary to consider and properly choose one of the many possible methods of physically moving the solid material through a vessel in which the retorting is to be carried out as well as the many other variances in operating parameters, all of which are interrelated. The choice of a particular method of moving the solids through the vessel must include a consideration of mechanical aspects as well as the chemistry and the processes involved. Further, it is necessary to consider the many possible sources of heat that may be used for the pyrolysis or destructive distillation.
In order to achieve a retorting process that is economically attractive and one which produces the maximum amount of high-quality shale oil, the various operating parameters must be controlled so that the overall process is economical, continuous and highly reliable. Any equipment usable in the process must permit a high throughput of material since enormous quantities of the shale must be processed for a relatively small recovery of shale oil. Process equipment for shale must have a high thermal efficiency and as in the case of all mechanical devices, the retorting equipment should be as simple as possible so that relatively proven and economically attractive mechanical devices may be utilized in the operation of the retort.
In an effort to provide an economically commercial process literally hundreds of retorting processes have been proposed, each of which offers a somewhat different choice and/or combination of the many possible operating conditions and apparatus.
One problem with many prior art processes is that the quality of shale oil obtained is relatively low. In many shale retorting processes, large quantities of shale fines produced in crushing the shale and fines produced during the retorting process find their way into the condensed shale oil. These fines lead to costly and difficult separation problems. Also, a second common problem with many prior art processes is that the high temperatures required to volatilize and distill off the shale oil also lead to many secondary and frequently undesirable side reactions which may increase the production of the normally gaseous products and decrease the yield and quality of the condensable products. Another problem with many prior art retorting processes is that the retorting takes place in the presence of molecular oxygen which leads to decreased yields and an inferior product.
The quality and yield of shale oil produced is greatly dependent upon how the retorting process is operated. For example, the raw shale can be heated rapidly or slowly and the shale can be finely divided or be in widely varying sizes. These and many other factors greatly influence the quantity and quality of the shale oil produced and the overall thermal efficiency of the process. In essentially all processes for the retorting of shale, the shale is first crushed to reduce the size and time necessary for the retorting process. During the crushing or mining of the shale it is difficult to obtain uniformly sized pieces and/or costly to separate the crushed shale into various sizes. It is therefore desirable to have a retorting process which can accommodate a wide size range of solids.
Cylindrical-shaped retorts with helical or screw-shaped-type conveyors are known in the art as shown, for example, in U.S. Pat. Nos. 759,988; 1,388,718; 1,475,901 and 2,934,476. Screw-type conveyors or mixers are highly efficient for moving and mixing solids, but these patents teach processes wherein the retort is externally heated.
The use of direct contact solid heat transfer materials is also known in the art as shown, for example, in U.S. Pat. No. 2,788,314.
The use of stripping gases flowing in countercurrent flow relative to the movement of the solid being retorted is also known in the art as shown, for example, in U.S. Pat. Nos. 2,664,389 and 2,934,476. However, these countercurrent flow processes involve passing the stripping gas through the entire length of the retort and involve the condensation of hydrocarbons in the retort.
Many of the foregoing problems are solved by the process of the present invention which is designed to produce the maximum amount of condensable hydrocarbons with a minimum of gas yield and a minimum of volatile matter being left in the retorted solid.